For background information relating to wellhead assemblies and chokes (or sometimes referred to as valves) thereof, intended to control the flow from the well, reference may be had to the following US patents:
U.S. Pat. No.InventorTitle3,155,401MusolfWell Head Assembly3,297,344HanesConnectors For Well Parts4,194,718Baker et alChoke4,102,401ErbstoesserWell treatment fluiddiversion with low density ball sealers4,606,557CoffeySub-sea Wellhead Connector4,898,235EnrightWellhead apparatus for use with a plungerproduced gas well having a shut-in timer,and method of use thereof
Natural gas, produced from a subsurface or sub-sea gas producing formation (hereinafter subterranean formation), requires the separation of components that are normally liquid or that have relatively high condensation temperatures. These components, which are collectively referred to in the claims and the description with the expression “the condensables” include water, propane, butane, pentane, propylene, ethylene, acetylene and others such as carbon dioxide, hydrogen sulfide, nitrogen gas and the like. Typically, the gas stream is treated, on surface, downstream of a wellhead that is connected with a subterranean gas producing formation via a primary wellbore containing a tubing extending downhole from the wellhead.
Separators to remove water from gas as it is being produced are known, for example in U.S. Pat. No. 5,333,684. This device uses floating balls that float up and block a flowpath when a water level in the wellbore becomes high, and then as gas pressure builds, and forces the water level down, allowing production of gas that is free of liquid water. This device is only capable of keeping liquid water out of produced gas. It is not capable of neither removing water from the wellbore, nor from lowering the dew point temperature of the produced gas.
U.S. Pat. No. 5,794,697 also discloses a downhole separator for taking gas from a mixture of liquids and gas produced into a wellbore. This patent focuses on downhole compression of the gas and re-injection of the gas into a gas cap over the oil remaining in the formation. A separator is shown and described as a auger that imparts a swirling motion to the fluids, and then removal of the gas from the center of the swirl. This separator also does not lower the dew point temperature of the gas, but only separates existing phases.
European patent application 0711903 and U.S. Pat. No. 3,599,400 disclose centrifugal oil/gas separators in which the produced crude oil and natural gas are separated by centrifugal forces but in which the produced phases are not expanded so that these separators only separate existing oil and gas phases.
Separators that are effective to lower dew points of gases generally require complex equipment and instrumentation, such as refrigerated sponge oils or glycol absorbers. Such operations are generally too complex to be placed at wellheads such as sea floor wellheads, and too expensive to be place at individual wellheads in a gas producing field.
It would be desirable to have a dehydrator as part of the wellhead assembly, downstream of the wellhead choke that not only removes liquid water, but that lowers the dew point temperature of the produced gas, and is simple and inexpensive.
Numerous methods and devices exist for separating components from gaseous or other fluids. Examples of conventional separation device include distillation columns, filters and membranes, settling tanks, centrifuges, electrostatic precipitators, dryers, chillers, cyclones, vortex tube separators and adsorbers. In addition, various inertia separators have been described in the art, equipped with a supersonic nozzle.
JP-A-02,017,921 refers to the separation of a gaseous mixture through the use of supersonic flow. The device includes a swirler positioned upstream of a supersonic nozzle. The swirling fluid stream then passes through an axially symmetric expansion nozzle to form fine particles. The swirl is maintained over a lengthy axial distance, creating a large pressure drop.
U.S. Pat. No. 3,559,373 refers to a supersonic flow separator including a high pressure gas inlet, a rectangular-shaped throat, and a U-shaped rectangular-cross sectional channel. The channel includes an outer curved permeable wall. A gas stream is provided to the gas inlet at subsonic speeds. The gas converges through the throat and expands into the channel, increasing the velocity to supersonic speed. The expansion of the flow in the supersonic region results in droplet coalescence and the larger droplets pass through the outer permeable wall and are collected in a chamber.
UK-A-1,103,130 describes a method and device for the separation of components of a predominantly gaseous stream, wherein the steam is accelerated to supersonic speed and subjected to an intense electric field in combination with an intense sound. Natural gas from a well connected via a line may be treated in this manner.
EP-A-0,496,128 refers to a method and device for separating a gas from a gas mixture. The device includes a cylinder which converges to a nozzle and then diverges into a swirl zone. Gas enters an inlet port of the cylinder at subsonic speeds and flows through a converging section of the nozzle. The flow expands out of the converging section into the diverging section of the cylinder at supersonic velocity. A pair of deltoid plates impart a swirl to the supersonic flow. The combination of the supersonic velocities and the swirl assist in condensing and separating a condensed component from the gaseous components of the flow stream. An outlet pipe is positioned centrally within the cylinder to allow discharge of the gaseous components of the flow stream at supersonic velocity. The liquid components continue on through a second diverging section, which drops the velocity to subsonic, and through a fan, ultimately exiting the cylinder through a second outlet.
WO 99/01194 describes a similar method and corresponding device for removing a selected gaseous component from a stream of fluid containing a plurality of gaseous components. This device is equipped with a shock flow inducer downstream of the collecting zone so as to decrease the axial velocity of the stream to subsonic velocity. Application of a shock wave in this manner results in a more efficient separation of the formed particles.
These references describe various supersonic inertia separators. However, none describe or hint at their use as part of a wellhead assembly downstream of a wellhead choke.